Japanese Patent Laid-Open No. 2003-261808 discloses an oil-based inkjet ink which contains a solvent comprising 60% by weight or more of a polar organic solvent, and the polar organic solvent comprises 10% by weight or more of an ester solvent, whereby the ink is improved in storage stability.
In addition, Japanese Patent Laid-Open No. 2005-350563 discloses an ink which contains a solvent comprising predetermined amounts of an unsaturated fatty acid ester solvent and a hydrocarbon solvent, whereby the ink hardly causes clogging in an ink nozzle of an inkjet head even after the ink has been left for a long time in a state of being filled in the ink nozzle until it is used for jet printing.
However, the ink containing such an ester solvent is problematic in that the ink is too wettable to the nozzle plate of the inkjet ink head, and thus the ink attracted to the nozzle plate falls dropwise onto paper to stain printed images, or the ink attracted to the plate hinders ejection of droplets of the ink.
It is conceivable to add an additive to the ink in order to change the ink in wettability to the nozzle plate. However, in general, when the wettability of the ink is lowered, there is a risk that the ink becomes lowered in dispersion stability of pigments, in other words, storage stability. Thus, both have been considered to trade-off each other.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-261808.    [Patent Document 2] Japanese Patent Laid-Open No. 2005-350563.